This part is going to cover the build of the backbone, seat wishbones and rear wishbones. The seat wishbone to backbone cut is difficult due to the steep angle involved. The tube in the picture to the left has been marked for the angle (silver line). Note how the cut starts from the front I.D. of the tube (if started from the OD of the tube, a sharp edge would result and have to be sanded of flat again). The cut was started in a drill press using a hole saw and tube-notcher. The hole saw bottoms out before the tube can be cut through. Since the tube is round and the cut is also round, the cut does not follow the marked angle. This excess material forward of the angle line wraps and conformed to the backbones.
The picture on the left shows how the excess material has been rough cut with a hacksaw (lower piece of tube). The upper tube has been cleaned up by sanding a nice radius from the front seat wishbone ID point to where the hole saw stopped cutting. Laying the tube next to the backbone (center tube in picture) shows how the fit is progressing. A 10” half round file works well to final fit the cope.
After the seat wishbones are trimmed to fit the backbone, the rear ends are trimmed and coped to fit the rear wishbones assembled in Part 1. The parts can be assembled flat as shown to the left for a typical straight back chopper frame. Or, the connections can be done at angles to create a drop seat frame or dropped backbone for a drag style bike. By just changing he fitting of the copes, a number of frame designs can be made from this simple set of bends. In any case, it is a good idea to build a simple jig from wood to hold everything in place for tack welding. Blocks of wood screwed to a sheet of MDF or Plywood makes an adequate jig to hold everything in place.